Pulmonary congestion is a clinical condition that is caused by a number of different diseases such as heart failure or kidney disease. It consists of an accumulation of fluid in the interstitial and alveolar space of the lung following increased blood pressure in the pulmonary capillary vessels that leads to leakage of water from the blood to the lung space. It causes fluid retention and fluid redistribution in the body and leads to symptoms like dyspnea, fatigue, and activity intolerance. Pulmonary congestion resulting from elevated left atrial and left ventricular filling pressures is a main reason for heart failure hospitalization. This condition has a progressive nature and clinical signs and symptoms of pulmonary oedema occur late, typically when the lung fluid has increased at least six-fold from the initial stage of interstitial oedema. This means that pulmonary oedema is often not detected early, and necessary treatment for the patient is delayed.
Bio-impedance measurements, obtained by a bio-impedance monitor using, for example, external electrodes or an implanted device to measure the resistance of biological tissue to a small alternating current flowing across a region of interest, e.g. the thorax, can be used to detect pulmonary congestion. The principle underlying this technique is the fact that the electrical impedance (resistance and reactance) of biological tissue is directly linked to the hydration and water content of the tissue, namely intra-cellular and extra-cellular water. When thoracic fluid accumulates (e.g. during pulmonary congestion), there will be a better conductance of the current resulting in a decreased impedance. By measuring the impedance at different frequencies the resistance of the extracellular water (Re) can be estimated according to the Cole-Cole model. Therefore, measurements of the electrical properties of the tissue can indicate the amount of fluid present in that part of the body.
If external electrodes are used, impedance measurements are influenced by several factors including sensor placement, skin moisture, body dimensions and body posture. It has been found that body dimensions and fat mass are particularly relevant to thoracic impedance measurements, making such measurements subject-specific. Implantable devices are not affected by variations in electrode placement or skin moisture; however, it is known that the measurements made by such devices have variability due to less controlled measurement conditions (the patient may be unaware that a measurement is being taken and so measurements may be obtained in a variety of situations which would cause differing fluid distribution in the body, such as lying down, sitting, walking, or exercising). These factors, combined with the normal variability of bio-impedance measurements, make it challenging to determine when an impedance measurement for a specific patient is abnormal and thus indicative of excess fluid accumulation. Furthermore, studies have shown that the deterioration of heart failure patients and related changes in bio-impedance can occur over a different timespan for each patient. When bio-impedance is monitored, this can mean that false alarms are often generated for patients who deteriorate slowly, and/or that alarms are not generated soon enough for patients who deteriorate quickly.
WO 2014/091426 describes an algorithm for detecting abnormal measurements which defines and continuously updates a subject-specific range of normal variability for bio-impedance, using the measurements obtained from a subject. However; this algorithm does not provide a way to account for the variability between patients of the deterioration timespan.
A more reliable means of fluid content monitoring would be a valuable tool to improve outcomes in heart failure hospitalizations and reduce healthcare costs. There is therefore a need for an improved method and apparatus that can more reliably determine whether a given sequence of abnormal bio-impedance values obtained from a given subject are indicative of deterioration in that subject's heart failure condition, and that can raise an alarm at an appropriate time for any given subject. Such a method and apparatus could be used in a home or hospital-based monitoring system to detect the presence and progression of pulmonary congestion, as well as for monitoring improvements in the patient's condition as a result of receiving treatment. Such a method and apparatus could also be used in monitoring other physiological characteristics of a subject, such as weight, heart rate, blood pressure, temperature, etc., where low or alternatively high values of the physiological characteristic indicates that the subject has (or the degree to which the subject has) a physiological condition.